0号元素（英语：Neutronium），有时又被称为中子元素（英语：Neutrium），是指原子中仅含中子，不含质子的一种元素，或纯粹只由中子组成的物质。1926年物理学家安德利亚·冯·安德罗波夫发明了这个词，那时甚至还没有中子的概念。安得罗波夫将0号元素放在了元素周期表最开始，以代表其质子数比氢还要少。

然而，该术语的含义随着时间发生了改变，从20世纪后半叶起，这个词被用来指一种密度极大的物质，最早被用于科幻小说中，代表一种密度极大的奇特元素，直到在中子被发现后，0号元素已主要指代中子星内部存在的一种高密度、无质子的元素，目前多以多中子核物质来表示许多中子聚集在一起所形成的核素，这种物质目前仅存在于中子星内部。直到现在，这个词的使用尚有争议。

Neutronium (sometimes shortened to neutrium) is a proposed name for a substance composed purely of neutrons. The word was coined by scientist Andreas von Antropoff in 1926 (before the discovery of the neutron) for the conjectured "element of atomic number zero" that he placed at the head of the periodic table. However, the meaning of the term has changed over time, and from the last half of the 20th century onward it has been also used legitimately to refer to extremely dense substances resembling the neutron-degenerate matter theorized to exist in the cores of neutron stars; henceforth "degenerate neutronium" will refer to this. Science fiction and popular literature frequently use the term "neutronium" to refer to a highly dense phase of matter composed primarily of neutrons.